1. Field of the Invention
The invention relates to an optical device for reducing image errors and more particularly to an optical device with increased depth of field and method for fabricating the same.
2. Description of the Related Art
Improving optical imaging systems to obtain images that are free of errors or distortions introduced by optical devices used in optical imaging systems has long been a goal in the development of optical imaging technology. The errors or distortions introduced by the optical devices may include lens aberrations, such as non-rotationally symmetrical aberrations (e. g. coma or astigmatism aberrations) and rotationally symmetrical aberrations (e.g. spherical aberrations), and misfocus errors resulting from an object being located away from the position of best focus.
One approach to reduce such imaging errors or distortions introduced by the optical devices is to increase the lens F-number (which is the ratio of focal length to effective aperture diameter). However, increasing the lens F-number may reduce the optical efficiency of the optical imaging system. Another approach to reduce such imaging errors or distortions introduced by the optical devices is to increase the depth of field of the optical imaging system. Generally, a complicated lens system is capable of greatly expanding the depth of field, but is very expensive. Accordingly, it is desirable in the art to provide a simple optical imaging system with only one or a few lenses, which still has an extended depth of field.
U.S. Pat. No. 5,748,371 discloses an extended depth of field optical system using a specific phase mask that is located at one of the principal planes of the imaging system. Such a phase mask has been designed so that the optical transfer function (OTF) to remain approximately constant within some range from the in-focus position. However, since the phase mask needs to be implemented with a non-rotational surface with high accuracy, fabrication thereof is very difficult. Namely, fabrication errors increase for phase masks having complicated contours resulting in increased failures for subsequent image restoration processes, thus, increasing fabrication costs for camera module.
Therefore, there is a need to develop a novel optical device with increased depth of field capable of mitigating the above problems.